Polyolefin resins have many excellent properties such as chemical resistance and mechanical characteristics but have a drawback in that the polymers, because of being non-polar, exhibit low affinity for polar substances. A conventional approach that is used to overcome this drawback is to modify polyolefins by grafting polar groups derived from, for example, organic carboxylic acids with a carbon-carbon double bond to the polyolefins using organic peroxides as initiators.
Some example methods that have been adopted for such polyolefin modifications are one in which a modifier is added to a polyolefin and the polyolefin is modified by being extruded in a molten state at high temperature under high shear with use of a device such as an extruder (the melt method), and one in which a polyolefin is dissolved in a solvent and a modifier is added to the resulting solution to modify the polyolefin (the solution method).
In the melt method, polyolefins, in particular, those containing tertiary carbon atoms such as polypropylene, polybutene and polymethylpentene, are prone to be decomposed at the tertiary carbon atoms. Because the modification with a modifier such as an organic carboxylic acid having a carbon-carbon bond is a grafting reaction, the decomposition reaction occurs more markedly when a large amount of grafting is to be introduced into the polyolefin. Thus, the melt method encounters a difficulty in satisfying both the amount of grafting and the molecular weight.
In principle, the solution method can suppress the decomposition reaction by setting the reaction temperature to below the level used in the melt method. However, the solution viscosity is generally so increased under such low reaction temperature conditions that problems such as difficult stirring are caused. Thus, it is difficult to satisfy both the amount of grafting and the molecular weight even by the solution method.
Among studies on the satisfaction of both of the amount of grafting and the molecular weight, for example, JP-A-2006-328388 (Patent Document 2) discloses a method in which an isotactic polypropylene, an organic acid component, and a peroxide including a peroxycarbonate structure are mixed together in an organic solvent while performing heating so as to give an acid-modified polypropylene. However, polymers modified by this method cannot attain sufficient strength when formed into composites with carbon fibers, glass fibers, cellulose fibers, plant fibers or the like. Thus, the balance between the amount of grafting and the molecular weight is still to be improved.
Another known method for further enhancing the balance between the amount of grafting and the molecular weight is the solid phase method in which a polyolefin polymer is modified at a temperature that is less than the melting point of the polymer. The solid phase method is advantageous in that the modification reaction does not involve a shear force to the molecular chains associated with stirring and consequently the breakage of molecular chains by shearing is avoided and the decrease in molecular weight can be correspondingly reduced.
One of the problems of these known modification methods, in particular, the solid phase method, is that the grafting reaction occurs locally and the product tends to have a nonuniform quality and to contain gel. Another problem is that depending on the type of a polyolefin to be modified, the grafting reaction is accompanied by crosslinking reaction at times. Gelation by the crosslinking reaction or the like of modified polyolefins obtained by the known methods results in problems such as fish eyes in films. Further, composites of such polymers with carbon fibers, glass fibers, cellulose fibers, plant fibers or the like exhibit poor strength.
Regarding the solution method, some studies focus on solvents used for the modification reaction and attempt to solve the above-described problems by using a solvent cleaned of specific impurities. For example, JP-A-2010-18750 (Patent Document 1) discloses a method for producing an unsaturated carboxylic acid-grafted polyolefin with low gel content and little coloration. In the disclosed method, an unsaturated carboxylic acid is grafted to a polyolefin in purified 1,1,2-trichloroethane as a solvent that has been cleaned of alcohol compounds and/or epoxy compounds, at a reaction temperature of 40 to 130° C. and a reaction pressure of not more than 1 MPa. Patent Document 1 also discloses that an unsaturated carboxylic acid-grafted polyolefin obtained by such a production method contains a gel that is insoluble in 140° C. xylene but the content of such a gel is as low as less than 0.02 wt %, and further discloses that the amount of grafting by the unsaturated carboxylic acid is 0.1 to 10 wt %.